xslt 2.0 distinct-values, contains function failed

I get distinct @source attribute values of graphml file:

<xsl:variable name="sources" select="distinct-values(root()//gml:edge/@source)"/>

then, I try to check does this string contains search word or doesnt:

<xsl:value-of select="contains($sources, 'N')"/>

but, transformation fails with error:

Error in XPath 2.0 expression Too many items

Whats the reason?

PS: fn:matches leads to the same result (error)